tretman voda- adsorpcija

8/12/2019 Tretman voda- Adsorpcija

http://slidepdf.com/reader/full/tretman-voda-adsorpcija 1/12

Adsorption WA T

E R T R E A T M E

N T

WATER TREATMENT

pump

pump a t r a z i n e (

m g / l )

2

1.5

1

0.5

00 10,000 20,000 30,000

bed volumes (m3/m3)

influent

effluent

macro pore

micro pores

pesticides

meso pore



Framework

This module explains adsorption.

Contents

This module has the following contents:

1. Introduction

2. Theory

2.1 Equilibrium

2.2 Kinetics

2.3 Mass balance

2.4 Solutions for the basic equations

3. Practice

3.1 Pseudo-moving-bed ltration

3.2 Pressure ltration 3.3 Powdered activated carbon

104

WATER TREATMENT ADSORPTION



1 Introduction

Water contains dissolved organic matter that can-

not be removed with oc formation, oc removal

or sand ltration. These dissolved organic com-

pounds are:

- odor-, taste- and color-producing compounds

- organic micropollutants (pesticides, hydrocar -

bon compounds)

Activated carbon adsorbs (part of) the organic

matter and is mainly used to treat drinking water

produced from surface water.

In the past, drin�ing water produced from surfacerinking water produced from surface

water would pass through the following steps: ocformation, oc removal (sedimentation and ltra-

tion) and disinfection with chlorine.

This was sufcient to comply with the drin�ing

water guidelines for turbidity, odor, taste and hygi-

enic reliability.

In 1987 Amsterdam Water Supply discovered

the presence of pesticides (Bentazon) in drinking

water. Due to this discovery, the traditional treat-

ment of surface water was no longer satisfac-

tory and an extension with activated carbon was

required.

In addition, chlorine can react with organic matter

(precursor), and trihalomethanes (THMs) can be

formed. These THMs are toxic.

To reduce the concentration, the formed THMs can

be removed with activated carbon. The problem,

however, is that carbon is rapidly saturated with

THMs and has to be regenerated frequently. It is

preferable to prevent the THMs from being formed.This can be done by reducing the concentration of

the precursor before adding chlorine.

Precursors cannot be measured directly, but the

concentration of organic matter, expressed as TOC

or DOC, is an indication.

The best way to prevent THM formation is to avoid

the dosing of chlorine, but this requires another

treatment setup.

Activated carbon is a substance with a high carbon

concentration (e.g., pit-coal, turf).

Under high temperatures this material becomes

carbonated, meaning that the carbon partly trans-

forms into carbon monoxide and water. This is how

the carbon gets its open structure (Figure 1).

The internal surface area of the activated carbon is

several times larger than the external surface area.

macro pore

micro pores

pesticides

macro pores > 25 nm

meso pores 1 - 25 nm

micro pores < 1 nm

meso pore

Figure 1 - The open structure of activated carbon

The activated carbon lters of the drin�ing water

production plant at Kralingen have the objective

to improve the taste of the water, to reduce the

regrowth of bacteria in the piped networ�, and

to remove toxic substances from the water.

The carbon lters are placed after the oc for -

mation, sedimentation and rapid sand ltration

to avoid rapid clogging.

The installation is based on pressure lters, so

the construction height can be limited and only

an extra pumping phase (middle pressure) is

needed.

The installation has the following characteris-tics:

Filter bed height: 4 m

Filtration surface area: 28 m2

Number of lters: 12

Contact time (EBCT): 12 min

Type of carbon: Norit ROW 0.95

Regeneration frequency: 1.5 year

105

ADSORPTIONWATER TREATMENT



Hence, a majority of the adsorbed substances is

adsorbed inside the carbon.

The dissolved organic matter can be removed

from the water by ltration through a bed of acti-

vated carbon.

Organic matter diffuses from the water phase to

the surface of the carbon grains. The organic com-

pounds are further transported into the carbon to

be attached in the pores.

The adsorption of organic matter is not nite.

There is equilibrium between the concentration of

dissolved compounds in water and the quantity of

substances that are adsorbed onto the carbon.

When different kinds of organic compoundsare present in the water, competition will occur.

Compounds that have been well-adsorbed will

occupy adsorption places that cannot be used

by compounds that are less adsorbable. Large

organic molecules can also bloc� micro pores,

thus preventing the smaller organic molecules

from entering these micro pores.

After some time the activated carbon is saturated

with adsorbed organic matter and the carbon

needs to be cleaned. This is done by removing

the carbon from the installation and heating it to

1000 oC.

This regeneration process has to be carried out

once every couple of years.

Activated carbon lters operate in the same way

as rapid sand lters. Mostly downward ow, open

lters are applied to prevent ne carbon grains

from washing out (Figure 2).

Since the contact time is the most important param-eter for good removal, the lter is often designed

with high beds to reduce the construction surface

area. Therefore, activated carbon lters cannot be

operated under gravity, ma�ing an extra pumping

phase necessary. After passing the lter bed, the

water reaches the bottom construction of the lter

and is then collected and transported to the clear

water tank or the next treatment step.

When the lter is clogged with suspended matter

or biomass and the resistance is high, the lter

bed is backwashed.

The backwash water is drained by troughs that are

placed above the lters.

Clogging of the lter by suspended matter can

lead to a higher regeneration frequency. Therefore,

activated carbon lters are usually placed after

oc formation, oc removal and rapid sand ltra-

tion (Figure 3).

A

A cross-section A-A

Figure 2 - Longitudinal and cross-section of an acti-

vated carbon ltration installation

clear water reservoir

activated carbon filtration

rapid filtration

floc removal

reservoir

ozonation

floc formation

Cl2/ClO2

Chlorine dosage for transport = 0.3 mg/l

activated carbon filtration:- removal of organic matter

- removal of pesticides

ozonation:- desinfection

- oxidation of organic matter dosage = 2-3,5 mg/l O3

reservoir:- storage

- leveling off - auto purification

Fe(III)

Figure 3 - Treatment process at Kralingen (Evides)

Figure 4 - Activated carbon ltration at Andijk (treatment

of surface water)

106




After activated carbon ltration, disinfection ta�es

place to prevent biological growth in the piped

network. This process can consist of slow sand

ltration, UV-disinfection or dosing chlorine or

chlorine dioxide.

2 Theory

2.1 Equilibrium

Similar to aeration, equilibrium is established dur-dur-

ing adsorption..

The maximum loading (loading capacity qmax)

depends on the concentration of adsorbable mat-

ter in the bulk liquid (water). The higher this con-centration, the higher the loading capacity is.

The relationship between the loading capacity and

the concentration of adsorbable matter in the bulk

liquid is called the adsorption-isotherm.

The best known is the Freundlich isotherm:

n

max s

xq K c

m= = ⋅

in which:

qmax = loading capacity (g/kg)

cs = equilibrium concentration (g/m3)

x = adsorbed amount of compound (g)

m = mass of activated carbon (kg)

K = Freundlich constant ((g/kg).(m3/g)n)

n = Freundlich constant (-)

The Freundlich constants K and n are inuenced

by water temperature, pH, type of carbon and the

concentration of other organic compounds.

Using laboratory experiments the Freundlich con-

stants can be determined for a single substance

with a specic type of activated carbon.

In Figure 5 the results of a laboratory experiment

are represented.

When these graphs are plotted on a logarithmic

scale (Figure 6), the Freundlich constant K can

be determined from the intersection of the graph

with the y-axis. The slope of the line is equal to

the Freundlich constant n.

The higher the K-value, the better the adsorp-

tion.

In Table 1 the values of the constants K and n are

given for some known substances.

From the structure formula of a substance, the

adsorbability can be derived. In general, non-

polar substances are better adsorbed than polar

substances. Substances with double bonds will

be better adsorbed than substances with single

bonds.

phenol

chloro phenol

dichloro phenol

trichloro phenol

350

300

250

200

150

100

50

00 10 20

cequilibrium (mg/m3)

q ( m g / k g )

Figure 5 - Loading capacity as a function of equilibrium

concentration

phenol

chloro phenol

dichloro phenol

trichloro phenol

150

100

50

5 10 30

cequilibrium (mg/m3)

0

0

q(mg/kg)

Figure 6 - Logarithm representation of the Freundlich

isotherm

107




2.2 Kinetics

The kinetics equation (equation of motion) for

activated carbon ltration is as follows:

2 0 s

dc dcu k (c c )

dt dy= − − ⋅ −

in which:

k2 = mass transfer coefcient (d-1)

c0 = initial concentration of organic compound

(mg/l)

u = pore velocity of the water (m/s)

cs = equilibrium concentration of organic

compound linked to a certain loading of the

activated carbon (mg/l)

The kinetics equation consists of a convection term

with which transport of the compound through the

lter bed can be described::

dcu

dy

and a removal term:

2 0 sk (c c )−

The rate of mass transfer is similar to aeration pro-

portional to the difference between the prevailing

concentration and the equilibrium concentration.

The equilibrium concentration depends on the

loading and is determined by the Freundlich iso-

therm.

The lower the loading of the carbon, the lower is

the equilibrium concentration and the higher is the

mass transfer rate.

The mass transfer coefcient depends on the

compound to be adsorbed and the type of carbon

(including the grain size).

In addition, the mass transfer coefcient can be

inuenced by the velocity of the water passing

the carbon grains. The higher the velocity of thewater, the better the mass transfer is between

liquid and carbon.

2.3 Mass balance

In Figure 7 the activated carbon lter is schema-

tized as a cube, in which:

Q = ow (m3/h)

B = width of lter (m)

L = length of lter (m)

Compound K.

((g/kg).

(m3 /g)n)

n (-)

alkanes

CH3Cl 6.2 0.80

CH2Cl

212.7 12.7

CH2Br 44.4 0.81

CHCl3 (chloroform) 95.5 0.67

CHBr 3 (bromoform) 929 0.66

CH2Cl - CH

2Cl (DCEA) 129 0.53

CH2Br - CH

2Br (EDB) 888 0.47

CH2Cl - CHCl - CH

3 (1,2 DCP) 313 0.59

CH2Br - CHBr - CH

2Cl(DBCP) 6910 0.60

alkenes

CCl2 = CHCl (TCE) 2000 0.48

CCl2

= CCl2

(PCE) 4050 0.52

pesticides - organochlorides

Dieldrin 17884 0.51

Lindane (HCH) 15000 0.43

Heptachlor 16196 0.92

Alachlor 81700 0.26

pesticides - organitrogenes

atrazine 38700 0.29

simazine 31300 0.23

pesticides - fenolderivates

dinoseb 30400 0.28

PCP 42600 0.34pesticides - fenoxycarbonicacid

2,4 D 10442 0.27

2,4,5 TP 15392 0.38

aromates

C6H

6(benzene) 1260 0.53

C6H

5Cl 9170 0.35

CH5CH

3 (toluene) 5010 0.43

C6H

5NO

2 (nitrobenzene) 3488 0.43

C6H

5COOH 2802 0.42

C6H

5OH (phenol) 503 0.54

C10H8 (naftalene) 7260 0.42

Table 1 - Freundlich constants K and n of several

substances

108




dy = height of lter (m)

c = concentration of organic compound (g/m3)

An organic compound with a concentration c0

enters the system with a ow Q and leaves the

system with a concentration c1. The difference in

concentration between in- and outow is adsorbed

on the activated carbon and increases the loading

of the carbon.

The continuity equation or mass balance is:

dq v dc

dt dy= −

ρ

in which:

v = ltration rate = Q/BL (m/h)

q = loading (g/g)ρ = density of the carbon (g/m3)

2.4 Solutions for the basic equations

The system of equations is non-linear and cannot

be solved analytically.

When a stationary situation is assumed, and

when the inuent concentration and the ow are

assumed to be constant, the efuent concentra-

tion of the activated carbon lter can be calculated

using the Bohart-Adams equation. This equation

is derived from the mass balance and the kinet-

ics equation.

0 02

e

c BV c1 exp � EBCT 1c q

⋅ = + ⋅ ⋅ − ⋅ ρ

VEBCT

Q=

Q T TBV

V EBCT

⋅= =

in which:

EBCT = empty bed contact time (h)

BV = ltered water per bed volume (m3/m3)

T = lter run time (h)

V = volume lter (m3)

Figure 8 shows the progress of the organic com-

pound concentration in the activated carbon l-

tration.

Water with a concentration of organic compound

c0 is supplied. Since, in the beginning, the carbon

is not yet loaded, the efuent concentration of the

organic compound drops to zero.

After some time, the loading of the carbon

increases, the available adsorption places are

lled, and brea�through of the organic compound

in the efuent occurs.

BV=0

∆ t

2∆ t

3∆ t

H

co

Figure 8 - Progress of the concentration in time and

height

Figure 7 - Schematic representation of activated car-

bon lter

109




Finally, the activated carbon is saturated without

any removal of organic matter.

In Figure 9 the efuent concentration is plotted

against the lter run time (expressed in number

of bed volumes). This curve is called the break-

through curve.

In the beginning the efuent concentration is 0.

After time the efuent concentration increases

until the activated carbon is saturated and the

efuent concentration is equal to the inuent con-

centration.

When the efuent concentration of the activated

carbon lter no longer meets the standards, the

lter must be regenerated.

The run time of activated carbon lters depends

on the objective.

The brea�through of THMs occurs relatively fast

(15,000 BV); for the removal of taste substances,

however, longer run times can be applied (50,000BV) without intermediate regeneration (Figure

9).

Contact time

The correlation between contact time and lter run

time depends on the adsorption characteristics of

the compound to be removed.

In general the lter run time increases exponen-

tially with increasing contact time (Figure 10).

Hence, per cubic meter of activated carbon, a

larger volume of water can be treated before

regeneration is necessary. Application of a shorter contact time indicates that

a smaller volume of activated carbon is needed.

This leads to lower investment costs.

The regeneration costs, on the other hand, will

increase.

An economical optimum depends on the adsorp-

tion behavior of the compound to be removed.

3 Practice

To solve the above-given basic equation, a sta-

tionary situation is assumed in which the inuent

concentration is considered to be constant.

In reality, the inuent concentration is not constant

but varies. Hence, the brea�through curve will not

describe a perfect “S”-form (Figure 11).

If the inuent concentration is high, relatively large

amounts of organic matter are adsorbed because

300

f i l t e r r u n t i m e ( d a y s )

contact time (min)

250

200

150

100

50

00 10 20 30 40

Figure 10 - Relationship between contact time and lter

run time THM Bentazon taste

0 20,000 40,000 60,000

0.25

0.5

0.75

1

0

bed volumes (m3 /m3)

c e

/ c 0 (

- )

Figure 9 - Breakthrough curve

a t r a z i n e (

m g / l )

2

1.5

1

0.5

00 10,000 20,000 30,000

bed volumes (m3/m3)

influent

effluent

Figure 11 - Breakthrough curve measured in practice

110




of a large driving force. When, afterwards, a low

inuent concentration occurs and the loading of thecarbon is already high, the saturation concentra-

tion can be almost equal to the inuent concentra-

tion and adsorption is limited.

In extreme cases even desorption (higher concen-

trations of organic compounds in efuent than in

inuent) can occur.

This happens, for example, after terminating trans-

port chlorination. Before termination, THMs were

removed in the lters and accumulated in the acti-

vated carbon. After termination, THMs no longer

form and the inuent concentration is nil (Figure

12). In the efuent of the activated carbon lters,

THMs are still present because of desorption.

3.1 Pseudo-moving-bed fltration

A special application for activated carbon ltration

is the “pseudo-moving-bed” system (Figure 13),

where two lters are placed in a series.

After brea�through occurs in the rst lter, the sec-

ond lter ta�es care of the polishing, and the efu-

ent of the second lter still meets the guidelines.

The moment the second lter brea�s through, the

rst lter is regenerated and then connected after

the second lter.

The cleanest lter is thus always the last one

(Figure 14).

The advantage of this setup is that the storage

capacity of the lters is better used, resulting in

longer run times.

Disadvantages of this setup are the high hydraulic

loadings of the lters and the complex system of

pipes and valves.

3.2 Pressure fltration

c h l o r o f o r m C

H C l 3 ( m g / l )

0

0 20,000 30,000

bed volumes (m3/m3)

influent

effluent

10,000

10

20

30

40

50

transport chlorination stopped

Figure 12 - Occurrence of desorption after termination

of transport chlorination

t = 0 t = ∆T

filter 1 is regenerated

t = 2∆T t = 3∆T

filter 2 is regenerated

Figure 14 - Breakthrough curves for pseudo-moving-bedactivated carbon ltration

Figure 15 - Steel pressure lters with activated carbon

pump

pump

Figure 13 - Principle of pseudo-moving-bed ltration

111




When activated carbon lters are placed in pres-

sure vessels, an extra pumping phase can be

avoided (Figure 15). The pressure is then sufcient

to lead the water through the activated carbon

lters to the clear water tan�s.

3.3 Biological activated carbon fltra-

tion

Biological activity occurs in all carbon filters.

Bacteria that grow on the carbon will decompose

organic matter.

Even with pre-chlorination bacteria can grow,

because the residual chlorine is adsorbed by the

activated carbon.

When organic matter is pre-oxidized by ozone (a

strong oxidizer), biological activity is stimulated,

resulting in increased lter run times and increased

removal of organic matter. This is called “biologi-

cally activated carbon ltration” (Figure 16).

Because of the increased biological activity on

the carbon, the organic macropollutants (DOC)

will occupy fewer adsorption places in the car-

bon. Hence, more space is left for the (persistent)

organic micropollutants. Some persistent organic

micropollutants like pesticides can even be (par-

tially) biologically decomposed after ozonation.

In some seasons (e.g., summer), the biological

activity will be greater than in other seasons (e.g.,

winter).

Therefore, the adsorption process will be the domi-

nant process in winter. The organic matter that is

adsorbed in winter will be partially decomposed

biologically in summer.

This phenomenon is called bio-regeneration.

With biologically activated carbon ltration, quic�ly

degradable matter is formed that will stimulate

bacterial growth.

Breakthrough of this degradable matter (expressed

as Assimilable Organic Carbon(AOC)) must be

avoided to prevent the regrowth of bacteria in the

piped network.

In addition, bacteria can be eroded from the car -

bon and enter the efuent, thus increasing thecolony counts.

A disinfection step with UV-radiation or chlorine

will then be necessary.

3.4 Powdered activated carbon

In addition to granular activated carbon ltration

(GAC), powdered activated carbon (PAC) dosing

can be applied.

With powdered activated carbon, small carbon

particles (1µm) are added to water.

These carbon particles are so small that during

transport they do not settle.

When the water is in contact with the carbon par-

ticles (after some time), equilibrium between the

organic matter in the water and on the powdered

carbon will be established.

The particles will be removed afterwards by a sand

ltration step.

For a powdered activated carbon dosing, a mass

balance can be set up, schematically represented

in Figure 17 :

0 0 e ec V q m c V q m⋅ + ⋅ = ⋅ + ⋅

0 ee

e

c cmq 0 W

V q

−= ⇒ = =

in which:

m = mass of activated carbon (g)

r u n t i m e ( y e a r s )

00 100

contact time (minutes)

with ozone

without ozone

guideline

2

1.5

0.5

50

1

Figure 16 - Inuence of pre-ozonation on required con-

112




A powdered activated carbon dosage has theactivated carbon dosage has thecarbon dosage has the

advantage over granular activated carbon ltra-tion that there are limited investment costs: there

is no need for a ltration installation (with an extra

pumping phase); a dosing unit for the powdered

activated carbon together with a mixing tank iscarbon together with a mixing tank is

sufcient.

The removal of pesticides with powdered activatedactivated

carbon, however, is limited and the rapid lters are

quickly clogged. This results in a large backwash

water loss.

Granular activated carbon ltration has a more

intensive water/carbon contact, thus pesticides

(and THMs) are removed more efciently.

V = volume lter (m3)

W = dosing of activated carbon (g/m3)

Starting with the Freundlich isotherm, the efu-

ent concentration of an organic compound can

be calculated, or the powdered activated carbon

dosage can be calculated for a determined efu-

ent concentration.

It is assumed that equilibrium occurs and the car-

bon is maximally loaded.The loading capacity is determined by the pre-

vailing concentration in the reactor, and in a

completely mixed system this equals the efuent

concentration.

For two ideal mixers placed in a series, the loading

capacity of the powdered activated carbon in the

second tan� is lower than in the rst tan� because

the concentration in the second tank is lower than

in the rst tan� (Figure 18).

V c0

q0, m

ce

qe, m

Figure 17 - Mass balance of powderd carbon dosage

c1 c0

c1 c0c2

q1

q2

q1

Ideal mixer, 1 - step

2 - step

c1 - c0 = W · q1

c1 - c0 = -W1·q1 -W·q1

(mass balance)

Figure 18 - One and two mixers in series

Figure 19 - Powdered activated carbon dosing unit at

Scheveningen

Further reading

• Water treatment: Principles and design, MWH

(2005), (ISBN 0 471 11018 3) (1948 pgs)

• Unit processes in drin�ing water treatment, W.

Masschelein (1992), (ISBN 0 8247 8678 5)

(635 pgs)

113




114


tretman voda- adsorpcija

Documents